1. Field of the Invention
The invention relates to the design and production of lightweight structural components. Objects in which its application is expedient and possible are all large-volume lightweight structures in which an essential part of the bearing pressure occurs as area load via skin sheets and which are provided with stiffening elements for load distribution, load diversion, reduction of deflection or prevention of denting or buckling. Typically such cases of stressing are particularly marked in many lightweight structures that are acted on by a pressure difference between the outside and the inside of the skin sheet in addition to the structural load. The invention can be used particularly advantageously for aircraft structures, in particular for fuselage structures, but also for wing structures, engine intakes, pressure bulkheads, landing gear shaft covers, etc. Other fields of application lie in liquid tanks or gas tanks, pressure tanks or vacuum tanks, components of rockets and rocket engines and fuselage structures of lightweight watercraft.
2. Discussion of Background Information
Without restricting the generality, the prior art and the background of the invention will be explained by way of example based on the construction of aircraft fuselage structures. Usually aircraft fuselages are produced from riveted panels that are reinforced by riveted stiffening elements—respectively stringers running lengthwise along the fuselage pipes and ribs running in the circumferential direction. Typically a stringer comprises a suitably formed stringer head, a stringer bar and a stringer base resting on a panel base at an angle of 90° relative to the stringer bar, which stringer base is riveted to the panel base.
The stress on the panel/stiffening element structure is very complex due to the different load originations and the static and cyclical loads dependent on many parameters. In the design of aircraft fuselages constructed in such a way, preset static strength demands must be met, fatigue strengths taken into account and safety guaranteed with regard to different preset critical failure scenarios. In the embodiment of the aircraft fuselage as a riveted structure, these demands are taken into account through the location-dependent and loading-dependent selection of the thickness of panel, stringers, ribs, the shape and the spacing of stringers or ribs, the dimensions of panel base and rivets and rivet spacing, etc. The fact that weight-saving potentials in terms of construction methods have been largely exhausted and that the production of this type of differential structure is too expensive because of the limited riveting speed and, in addition, can hardly be improved on in qualitative terms, have a negative impact on the design of the conventionally riveted structure.
It is known, for example, from P. Heider: Lasergerechte Konstruktion und lasergerechte Fertigungsmittel zum Schweiβen groβformatiger Aluminium-Strukturbauteile in: VDI-Fortschrittsberichte, series 2: Fertigungstechnik, no. 326, VDI-Verlag Düsseldorf (1994) to replace riveting by welding the stringer foot to the panel base from both sides simultaneously by means of two lasers. In order to realize this connection with a sufficiently well-developed root of the weld seam and in a manner low in pores, it is necessary for both laser beams to produce a common melting bath. This is achieved in that the two laser beams, placed opposite one another, are focused on identical positions with respect to the joint. Hot cracks are thereby avoided, through the use of suitable wire-shaped weld fillers, such as, e.g., wire of the alloy AlSi12. Through the very low linear energy of the process and the energy input symmetrical to the stringer, the deformation is limited.
In another embodiment of this principle the construction of completely welded shell components is possible, including stringers, ribs, clips, distribution belts and rib heads. See, for example, P. Brinck et al.: Schalenbauteil für ein Flugzeug und Verfahren zur Herstellung, PS DE 198 44 035 C1.
Despite better static strength and higher rigidity compared with a riveted connection, the disadvantage of a connection produced in this way lies in its lower damage tolerance which is manifested by, e.g., a higher rate of crack growth of a circumferential crack after crossing the stringer and a lower residual strength. The reason for this is that, on reaching a welded-on stiffening element, a crack spreads out into the latter. Whereas with a conventional differential construction, the crack growth in the fuselage planking is delayed through the riveted or adhered reinforcements, such as stringers or ribs, since the crack tip does not spread into the stiffening elements for a certain number of load cycles and, moreover, is held together through the intact reinforcement, in the welded-on stiffening elements the crack grows in the planking and the stiffening element simultaneously, without a noticeable crack-delaying effect occurring. The weight-saving use of laser beam-welded fuselage shells is thus only possible for fuselage regions for which the design criteria for damage tolerance do not need to be met, i.e., only for the lower shells of the fuselage.
The reason for this defect or disadvantage is that the known integral embodiments of the connection of the stiffening elements does not provide any adequate geometric, stress-related or microstructural possibilities for stopping a crack, a less damaging crack branching or an energy dissipation near the crack opening. The crack can thus spread unhindered into the stiffening elements.
Another defect or disadvantage is that the direct tensile strength of a stringer/panel connection laser beam-welded from both sides simultaneously decreases with increasing weld seam depth, i.e., with increasing stringer thickness.
The reason for this is, i.a., to produce greater weld seam depths the weld parameters have to be changed such that a greater linear energy and more unfavorable ratios of wire conveying speed of the weld filler to welding speed have to be selected. Together with solid state mechanical influences, both of these lead to a greater under-matching in the welding zone, a broader overaged region in the heat affected zone and to an increased risk of formation of micro-hot cracks.
To improve the crack growth behavior of shell components with welded-on stiffening elements, it has become known from, for example, F. Palm: Metallisches Schalenbauteil, PS DE 199 24 909 C1 to increase the thickness of the bar of the stiffening element near the welding zone without increasing the connection depth of the laser beam weld seam made from both sides simultaneously. In other embodiments of the invention a reduced weld seam depth is made or notches placed between the weld seam and the increased thickness. The object of all three measures is to make it more difficult for a crack to spread in the direction of the stringer head. The crack can possibly be deflected and can run for a certain distance in the weld seam or along the weld seam.
The disadvantage of this solution is that this embodiment is only geared to the two bay crack type of stress, i.e., the bearing of a longitudinal or circumferential crack over two rib sections or stringer divisions. Both for the “tension in the direction of the head of the stiffening element” type of stress, such as occurs in the lower fuselage region and for ribs, and for the combined “bending with bending deflection crosswise to the stiffening element” and “tension in the direction of the head of the stiffening element” types of stress, as occurs in the areas of the fuselage loaded by transverse stress, the proposed solution leads to a reduction of the bearable loads or to a premature stringer or rib rupture.
The reason for the defect is that the two disadvantages of an integrally welded structure—the lack of an effective mechanism for delaying cracks and the locally increased crack growth rate in the weld seam—are combated only with disadvantageous consequences regarding loading capacity for other types of stress, or cannot be combated at all.
An embodiment of a welded arrangement of panel and reinforcing elements that achieves an increase in residual strength and thus is also intended to render possible the use of welded fuselage shells for the side and upper shell area of the fuselage is known from H. J. Schmidt (PS DE 100 31 510 A1). To this end reinforcements are applied to the stiffening elements before the laser beam-welding. The reinforcements can thereby be arranged as doubler plates or as tension bands.
The doubler plates comprise high-strength Al alloys or fiber-reinforced metal laminates and are attached by riveting or an adhesive bond. The doubler plates must thereby be an adequate distance from the weld seam, which distance is determined by the temperature field of the welding process. The tension bands comprise high-strength steel alloys or titanium alloys or fiber composites and are inserted and twisted into through bores that are to be made beforehand. Cross section reinforcements are provided in the lower bar area of the stiffening element to contain the through bores. Another variant provides embodying the lower bar area in a slotted manner, inserting the tension band through a mounting opening in the slotted lower bar area and connecting the inserted tension band to the tension band in a form-locking manner by compressing the slotted bar area and subsequently connecting it to the skin sheet by laser beam welding in a known manner.
An increase in residual strength is achieved through the crack-delaying effect of the reinforcements. This occurs in that the number of the load cycles necessary for the complete severance of the stiffening element is increased and the reinforcing element does not fail until after the stiffening element. Through the latter effect the reinforcing element can reduce the crack opening angle and reduce the tensions at the crack tip for the period between the failure of the stiffening element and the failure of the reinforcing element.
The embodiment of the weld seam itself is not changed with respect to the previously known prior art. This means that the stringer or rib foot is attached to the skin sheet across its entire width by a single weld seam, whereby the weld seam is produced by laser beam welding from both sides simultaneously.
One defect of the arrangement is that it is not suitable for improving the prior art with regard to the two critical stress types “tension in the direction of the head of the stiffening element” and “bending in the direction crosswise to the stiffening element.” The danger of static failure as a result of the separation of the stiffening element from the panel, in particular during transverse stress in the side shells, therefore remains.
The reason for this is-that the unchanged weld seam arrangement and weld seam vicinity cannot bear any greater direct tensile stress or bending stress crosswise to the longitudinal directions of the stiffening elements.
Moreover, it has a disadvantageous effect that the reinforcements of the stiffening elements do not reduce the local crack growth rate in the weld seam and its direct vicinity. This applies in particular to stress types such as, e.g., transverse stress in which there is a danger of a crack spreading along the weld seam.
The reason for this is that, for reasons determined by the process and the arrangement, they have to be installed at a distance from the weld seam at which their effect in terms of stress relief for the weld seam is very slight.
Another defect is that the reinforcing elements cannot be applied to or inserted in the stiffening elements in an economic manner.
The reason for this is that additional production steps, such as, e.g., riveting the doubler plates, adhering the doubler plates or drilling very long through bores are necessary to apply the reinforcements, which steps are in themselves very expensive or in part even more expensive than the riveting of the stiffening elements to the panel which is to be replaced.
Furthermore, the fact that the variant with inserted tension bands is not suitable for ribs has a disadvantageous effect. The reason for this lies in the impossibility of drilling a curved slot or of bending together the two side pieces of the slotted lower bar area after inserting the tension band in a plastic manner without damage to the materials or a permanent deformation of the entire stiffening element.